Telephone instrument circuit



July 25, 1933. E R MGAN 1,919,314

I TELEPHONE INSTRUMENT CIRCUIT 7 Filed Jan. 28, 1932 2 Sheets-Sheefl 1 +500 OHMS RES/STANCE Z0 z! 500 I000 '1500 OHMS C l-Oma/V- I Q D 500 a OHMS Jul 25, 1933. E. R. WIGAN 1,919,314 TELEPHONE INSTRUMENT CIRCUIT Filed Jan. 28, 1932 2 Sheets-Sheet 2 Fig. 2. Fig. 4.

Patented July 25, 1933 UNITED: STATES PATENT" OFFICE EDMUND RAMSAY WIGAN, or Lennon, ENGLAND, ASSlZ GhTOR T SIEME S no'rms & COMPANY LIMITED,

OF LONDON, ENGLAND TELEPHONE NSTRUMENT cmourr Application/filed January 28, 1932, s mino. 589,472, and in Great Britain February 7, 1931.

This invention relates in general to tele phone substation circuits and the principal object of the invention isto provide an improved substation circuit in which side-tone is reduced to a negligible minimum when connected to different lines having varying characteristics and impedances. Another object of the invention is to provide a new and improved substation circuit in which by choice of suitable constants for the circuit the side-tone level, while remaining low, may be varied with the line impedance in a selective manner. a

A further object of the invention is to provide an improved substation circuit having induction coilwindings anda balancing network in which two superimposed. currents may be produced in the receiver circuit by the operation of the transmitter in case of an unbalanced condition" between the-line and time is connected to the substation circuit.

It is known that side-tone maybe suppressed when the substation circuit is connected to a lineof a particular impedance, but when a given substation circuit is con nected to lines of varying impedances" the side-tone may vary considerably. Another factor affecting the reduction of side-tone is the "frequency of the currents involved. Therefore, if line impedance and instrument impedance vary with frequency, the reduction of side-toneis bound to be effected. In practice various line conditions and char} acteristicsare encountered anda connection to 'aisubstation circuit involves not only the local linebut varying junction or trunklines to complete a telephone connection.

It might be mentioned here that total elimination of side-tone is not desirable except in the noisiest situations-as the side-toneto some extentserves as a guldeto the telephone v7 96 cycles per second.

.duction coil windings connected user in regulating the loudness 'of-his speech,

and also the absence of sound in one ear gives a feeling of deafness. v

Referringnow briefly to. the drawings:-'

Fig. "1' shows the relation between the voltage applied at the microphone terminals and the current generated in the receiver'circuit for a particular circuit ata frequency of Fig. shows the loci of the points P and Qtaken'from'Fig. 1 as the frequency is varied for a particular-circuit. 1

Fig. 3 show'stlie effect on the locus'of Q when the characteristics of thedifferent parts of the circuit are varied.

The loci shown in Fig. 4 represent the movement of the head o-f the impedance Vector in different cases in practiceat varying frequencies. v p v Figs. 5, 6,' and 7 show three differentsubstation or instrument circuits having their in-- in different ways. i 3 a The twoloci shown in 8 and 9 show the effect of the difference between two circuits. I L W Fig. '10 shows asubstation'circuit andconnections in a desk instrument for anauto matic, telephone system. i I

Fig. 11 shows a substation circuit and connections wherein'a resistance -'-and a con-v denser is used to form'a spark quenching circuit for the dial impulse springs.

In the present invention circuits are1proposed involving a three winding'induction coil and a balancingnetwork, the receiver circuit being shunted across a part of the.

balancing network and the transmitter being shunted across a part of the network containing a winding of the induction coil and a condenser in series. For side-tone reduction the receiver and the third winding. of the induction coil are tapped off the whole or a part ofthe impedance in thebalancing network. Several modifications are shown in which-the windings of the induction coil are connected in different ways. i i

Referring now in particular to Fig; 5 it will be seen that the transmitter feedcurrent from the line flows through the winding L2 of the induction coil and the transmitter T. Duringreceptionof. signals there is a source of alternating potential across the line con- 7 I duetors tending to drive a current through a pathinfthe substation circuit over the upper line conductor through the transmitter T and windingv L2 to the lower conductor. The current passing through the winding L2 induces a current flow in the winding L3 which actuates the receiver R. 7

'During transmission and considering the transmiter T as a source of E. M. F., current flows out to the junction pointfbetween the windings L1 and L2 of the induction coil and then divides, part going through the winding L2out loverthe line and part through the winding L1 and the balancing network-comprising Z, TY, and C. Since the current flowing through these two windings are in opposite directions, the inductive eiiect upon the winding L3 is towards neutralization de pending upon the balance of the circuits. In case the two circuits are out of balancethen voltage will be induced in winding L3. In addition, since'there is current flow over the balancing network and since the terminals of the receiver circuit are bridged. across the impedance Z, an E. M. F. from the primary circuit is produced. in the receiver circuit. The induced E. M. F. and the'E. M. F. from the primary circuit may oppose or aid each other dependent upon the connection of the terminals of the receiver circuit across the impedance Z.

The characterlstics of the circuit shown in Fig. 6 are substantially the same as that shown in Fig. 5.. induction coil is connected in series with the impedance Z and then bridged across the receiver circuit including the receiver R and Winding L3. v The characteristics of the circuit shown in Fig. 7 are substantially the same as, that shown in Fig. 5. The induction coil'winding L1 in this case has been reversed with respect to the winding L2 and the winding L1 is now' included in the transmission bridge including the transmitter T and winding L2.

When the substation circuit is to be installed in a certain location, the average impedance of the lines in use therein will be approximated and the windings of the induction'coil will then be connected in such a manner as to give the best results as regards side-tone reduction. a

7 Mathematical considerationof the circuit relations are given with a view to facilitating design so that'a maximum output and the inherent advantages of side-tone suppression can be obtained. I

I have investigated mathematically the variation of side tone with line impedance where i is the side tone current due to a voltage e enerated'in the microphone when the 'lineimpedanceis equal to Z;;+AL.

The L1 windingof the Z is that value of line impedance which gives zero side tone at the frequency considered.

AL is the vector difierence between Z, and

the line impedance under consideration.-

Z0 is the impedance of the instrument circult measured. from the hue terminals with the microphone and receiver connected.

PLa is a voltage step up ratio equal to the ratio of the open circuit a. c. voltage at the line terminals to an E. M. F. applied at the microphone ter1ninals.- v

KL 0 is the current step-downratio, equal to the ratio of the receiver current to the incoming line current.

Z0, PL a and KL@ can be ascertained by measuring methods in a particular instrument. r i

The equation shows that the side tone current is related as follows: (1) to the product of the transmission and reception eificiency.

of the circuit. The transmission efiiciency is proportional to I I Pia (Zcv Z AL) and the reception efiiciency to K40 (Z0 ZL AL) line impedance result in relatively large changes in side tone.

Fig. 1 should be referred to which depicts I a particular case experimentally confirmed) at a particular frequency of 796 cycles per second. Values along the abscissa axis from the point O represent resistance component values of line impedance. The values along 7 the ordinate axis through 0 represent reactance component values of the line impedance. Positive values are above the abscissa' axis, negative below. The value. of

Z0 for the instrument circuit is given by P0 and 0Q represents the line impedance for which the side tone is zero.

If the instrument circuit is connected to a line the impedance is represented'by the point'R, the impedance being OR then AL is represented by the line QR and the ratio AL (Zc+Z AL) is given by the ratio of QR to PR.

all

Fora given constant ratio of lengths QR" toPR-the locus of R is a circle although the circle may have infinite diameter as evidenced by the line locus AA which is at right angles to the line PQ. Any particular locus shows how AL may vary whilstthe side tone current remains constant although altering in phase. The values such as0.2 m. a/V indicate for the particular circuit considered the receiver current is 0.2 millianiperes per volt a. c. generated at the microphone. A value. of 0.5 appears to be tolerable. A value over 0.6 becomes annoying and'a value below 0.2 gives a sensation of deafness to the user. A 600 ohm non-reactive junction line connected to the instrument through a stone bridge would provide a line impedance of 600-1200 ohms approximately. The side tone ratio with zero local. line resistance would be about 0.1 In. a./V. Vith 450 ohm local line the side tone ratio would be about 0.2 m. a. /Vall taken at 796 cycles per second.

It has to be remembered that due to the drop in feeding current in a common battery circuitwith increased line resistance, the a. c. voltage falls by about'50% between zero and 450 ohm line resistance so that the s'idetone level would be the same in both cases.

Again for a particular circle. for instance that on which R is situated the ratio of Given one circle and. the value of milliamps per volt the circles 'for other values of milli-amps per volt can be drawn.

The radius of the circle is or in other words S (Z0 ZL) W From the above results it appears that the larger the value of Z +Zc for which the instrument circuit has no side tone, the greater the area in the locus diagram, Fig. 1,.for which side tone is below a given value.

T Again for a particular instrument circuit it may be ascertained whetherthe line im pedances of the lines with which the instrument maybe used entails too much side tone and an adjustment of the circuit made accordingly.

'The voltage and current parameters P L a and KL 9 'may' be ascertained by direct measurement. I

-Whilst an instrument circuit :with antiside tonearrangements may'giverise to no side tone when connected -to a line of a particular impedance and the currents involved are of a specified frequency, it does not follow that this occurs at other frequencies In this connection Fig. 2 should be referred to which shows for a particular instrumentcir cuit the loci of the points P and Q,- as thefre- I quency is varied. The direction of increasing frequency is indicated'by the arrow heads. The four pointsindicated refer to frequencies of500 796, 1592 and 3184 persec The locus forms of Fig. l-remains circularwith frequency variation. It will be clear that with the instrument circuit which gives the 'loci'shown in Fig. 2, .side'tone may be ex pected if the head of the line impedance vec tor does not move with Q, i. c. it must have the same value and phase angle as'the vec tor 0Q.

Figs. 5, 6 and 7 represent three ment circuits included in the present invention. In these figures, the receiver is designated R, the transmitter by T. The line terminals are denoted by L, C is a condenser, Ry is a resistance and Z an impedance. The bell is omitted but would be as is usual Connected between one line branch and the con denser, the condenser being a necessity in the case of common battery working.

The induction coil has three windin s de-' noted L1, L2, L3. The letters 8 and f enote the start and finish of windings. and finish of coil L3 is not shown for a reason which will later appear. being wound in the same direction'round the core the terms start and'finish have the well understood meanings. I v

The impedance Z, resistance By and condenser C is a unit which playsa major part in determining the Z value of the instrument circuit. 'R'y and Z maybe tapped'so that L3 andthe receiver R'may be shunted across a part or the whole of the series connection of the two.

With the circuits shown in Figs. 5, 6 and '7 the form of the locus of Q of Fig. 1 when frequency varies depends among other things on the'valu'es of C, Ryand Z. v If this coni nection contains only capacity and resistance, locilsuch as B, C and D in Fig. 3 are obtained which have their upper limits on. the resistance axis.

ing L3 and. finish of winding=Ll are coninstru- The start The windings Inthe case of B the start of windnected together. Locus D may be obtained by using a winding L3 of comparatively low inductance. The locus C may be obtained by reversing the connection of winding L3 that is by connecting its finish end to f of winding L1. I

Loci which cross over the resistance axis such as E and F are produced when the impedance Z contains inductance orcapacity. For locus F Ry is comparatively large.

The single point G results from a condenser and resistance of special values in the balancing part. The locus H is produced in cases in which there appreciable leakage inductance in the induction coil windings and is a modification of the locus D. The locus Hhas certain advantages- Clearly by suitable design of the instrument circuit the movement of Q in Fig. 1 when the frequency changes may be varied considerably and offers the possibility of suiting the instrument to lines on which it may be used.

Fig. 4: shows graphs of cases which occur in practice. A locus here such as L shows the movement of the head of an impedance vector with frequency. As to any one locus shown the points which .are encircled refer as in Fig. Q'to frequencies of 500-, 796, 1592 and 3184 cycles per second. The impedance vector, of course, has one end at O and its head on a locus.

The locus J is the locus of the head of the impedance vector for a long'length of standard cable in series with a 150 ohm local line. The locus K represents the same circuit in whiehhowever is included a stone feeding bridge with 'a 2 m.f. condenser in each Wire. LocusL refers to a long junction line of 150 lb; aerial wire with a local line of 150 ohms. Locus M refers to a case in which the impedance which faces the instrument is that of a non-junction connection between two subscribers using similar instrument circuits on the same automatic exchange the local line resistance being assumed to be 150 ohms to each instrument. the point Q for an instrument circuitwould have to move coincidently with the line impedance on the locus concerned in Fig. 4. In any case a divergence is-permissible and in fact desirable as exact co-inc-idence, i; e. no side tone, as noted before has a dlsturb ng efiecton the user. -q

No one instrument circuit can fit with the several loci of Fig. 4 and for an instrument of general use, an average locus such as N may be taken into account and the instrument designed with this in View.

Having pointed out the important factors influencing the design of the anti-side tone circuit of the present invention, certain.

formulae will be given for the circuits of Figs. 5, 6 and 7. These formulae indicate: the

value of ZL for an instrument, that is the line To keep the side tone zero impedance for which side tone is zer0,:and its variation with frequency, in other words an equation is given for the Q locus. Adjustments of the constants within the equationsmay be necessary to give thejgreatest elec trical efiiciency as regards speech transmission but electrical efiizciency may be largely ignored provided that the side tonelevel is low. The. practical value of the circuits is very closely related to the latter factor as by satisfactory side tone reduction a; relatively silent background is provided against which received speech is contrasted and simplicity and cheapness maybe given d-ueconsidera tion in design.

Certain assumptions have been made in deriving the equations',for instance theequa-, tion relating to the Fig. 5 circuit is .given on the assumption that the microphone will have a resistance much lower than the impedance of the lines to which the circuit would be connected.

Consequently the windings L1 and L2 are connected as an auto-transformer giving a step up of voltage fromthe microphone to the line. The size of the winding LS'hasan effect on the shape of the Q locus but may be arranged to give good reception efficiency. lVindings L1 and L2 in Figs. 6 and 7 act also as step uptransformers L2 in Fig. 7 having more turns than L1. I

It may be noted that the increase of effective resistance of the induction coil windings with frequency disturbs the equations except where L1 to L2 is unity when its effect is reduced. v p

In the equations it is convenient to refer all the winding ratios to the receiver or L3, winding. It is assumed that the ratio of turns will be proportional to. the square root of the ratio of inductances and that the coupling For the circuit of Fig. 5

In these equations:

coefficients between windings are unity. V I

tioned.

= T2 L1,. L2, 133 being the inductances of the three windings shown W=21rimes the frequency, 9' is the operator'J-l. f

The plus or minus signs are used according to the connection of the receiver winding L3. The upper sign is used when the finish of L3 is connected to the receiver and the lower (minus) sign when itis the start that is connected to the receiver.

If T1=T2=1 for the case covered by Equation (1) according to whether the positive or negative sign is used.

In Equation (5) the numerator and denominator can each represent an impedance equivalent to a condenser and a resistance in series. Such a circuit arrangement can be adjusted in such a wa that the impedance ZL is independent of requency, that is, be: comes a pure resistance. The arrangement is then aperiodic. Each Equation -1, 2 and 3 allows of an aperiodic arrangement. In the case of Equations 1 and 3 thelower sign of the two alternative signs is used. In the case of Equation 2, thelower sign is used f T1 is less than unity and the upper sign 1f T1 is greater than unity.

The adjustment required of the constant may be gathered from Equation (1) If ZL is to be independent of frequency the numerator anddenominator of the fraction must have the same angular value at all frequencies. v

Equatmg tangent values This value of th Fig. 3.

ZL gives the locusG in The resistances of the induction coilwind-- ings are not'taken account of in the equations Ry, Z and C are the elements before men 7 for the reason that they are designedly low and although they may rise with frequency do not disturb the equations to an extent greatly influencing design. I The effect of leakage inductance is to add to ZL a negative rcactance equal to the positive reactance of the leakage' inductance at each frequency. L

To meet the special case of the line for which the graph or locus M is given in Fig. 4 the circuit-of Fig. 5 may have the'following values: T1T2='1, Ry=200 ohms, Z== ohms resistance with 10 milli-henries inductance, 0 2 microfarads, L3=93 milli-henries. The sign of connection of receiver winding is positive. 7 v

The induction coil, windings may have a d. c. resistance of about 20 ohms each.

The locus of Q is shown'in Fig. 8- for this case. The locus'marked K gives thempedance locus of a circuit of the same im pedance (e.g. a distant twin instrument) measured with .150' ohms in series, andseen through a Stone bridge with 150 ohms on the outgoing side. The two loci ,overlap but there is no great divergence at the main speech frequencies.

The case represented in Fig. 4 by .the graph or locus J can be met by another arrangement the form shown in Fig. 5.

The circuit is made up with the following L3=36 milli-henries. The sign of connectionof the receiver winding is negative.

. The locus of Q, here isofthe form marked H in Fig. 3. This may be comparediwith the locus J of Fig. 4 which concerns a cable standard 20 lb. wire asmeasured from the end of. a ohm loop through a repeating coil exchange circuit. The two' loci are shown in Fig. 9. The inflexion in the locus H is due to leakage inductance in the induction coil. a f

A circuit giving good all round performances has the Q locus shown in Fig.2 and the constants are as follows: T2, T1 =1, Ry -525 ohms, 0 2 m. f.,, Z =75 ohms, -L3=93 milli-henries. The. circuit is; as in Fig. 5 and the sign of connection of the receiver winding is positive. 1

In the foregoing examples the receiver impedance was 124+j .230 ohms-at 7 96 c. p. a. The transmitter had a resistance of the order of 50 ohms and the induction coil windings had an a. c. resistance of the order of 20 ohms. I The resistance Ry may however be included in the Winding of L1 in the cases of Fig. 5 by winding the coil with a high resistance alloyor'with-small gauge copper the case of Fig. 6. o v p Both resistance Ry and impedance Z may be tapped so that bothmay' be adjusted. to

wire. Resistance in Z may go in coil'Ll in suit the'line with which the instrument may be used.

It. is pointed out-that as will be seen in the equations the use of the resistance Rg (as considered separate from Z) gives con-' siderable and simple control of the position of the Q, locus on the diagram as distinct from the shape. In place of By an inductive re sistance may be used. The Equations (1), (2) and are general and the effect of any such substitution will be perceived. The ct fect of the presence or absence of the condenser C willalso be perceived.

Three conductor cords CK are used for the connection'between separate parts. DL is the dial and DLa and DLb are dial off-nor- -mal contacts. Switch hook. contacts are situated at X and are closed when the instrument is taken into use. j 7

As to the induction coil the same designations L1, L2, L3 are used as in Fig. 5v The resistance Ry is formed by the high resistance winding L1. The impedance Z may be resistance or resistance and reactance.

Thefresistance (Ry) may however be associated with the condenser in the bell box and if desired may be included so as to be in series with the bell or condenser The talking circuits of Fig. 10 are substantially the same as those described for Fig. 5 and therefore need not be described in detail.

Fi 11 shows a case in which the resistance Iiy is used in conjunction with the condenser C to form a spark quench for the dial impulse contacts. The talking circuits of Fig. 11 are somewhat similar to those described for Fig.5. When the calling device DL is operated the dial off normal springs DLa and DLZ) .close to provide the impulse circuit which may be traced as follows: from the upper line L, impulse springs DL, oil normal springs DLa and DLZ), the lower switchhook springs X and to the lower line L. The circuit for quenching the spark at the impulse springsDL may be traced from springs DL, condenser C, resistance By, and off normal springs DLa .back to the impulse springs DL. I

It is essential to the invention-to have two impedances such as (C and By) andZ in series.

I desire to state however that as regards the circuit arrangement shown in Fig. 5, I makeno claim for such circuit when Ry is zero, C has a finite value and the impedance Z is a pure resistance.

What I claim-as new and desire to secure by Letters Patent is: I

1. In a substation telephone circuit comprising a transmitter, a receiver, two imp'edances connected in series, an induction coil having three windings, a line, a series circuit including the second of said windings and said transmitter in series connected in bridge of said line, a second series circuit including said two impedances-and the first of saidwindings in series connected in bridge of said transmitter, and circuit means connecting the third of said windings in series with said receiver across one of said impedances. I

2. In a substation telephone circuit com prising a transmitter, a receiver, two impedances connected in series, an induction coil having three windings, aline, a series circuit including the second. of said windings and said transmitter in series connected in bridge of said line, a second series circuit including said two impedances and the first of said windings in series connected in bridge of said transmitter, and circuit means for connecting the third of said windings in series with said receiver in bridge of that portion of said second series circuit including one of said impedances and said first wmding.

3. Ina substation telephone circuit comprising a transmitter, a receiver, two impedances connected in series, an induction coil having three windings, a line,a series circuit including the first and second of said windings and said transmitter in series connected in bridge of said. line circuit, a second series circuit including. said two impedances in series connected in bridge of that portion of said first series circuit including said first winding and said transmitter, and circuit means connecting the third of said windings in series. with said receiver across one of said impedancesf 3 i 7 4. A substation telephone circuit. comprising a transmitter, a receiver, a first impedance including a resistance and a condenser in series, a second impedance connected inseri es with said first impedance, an induction coil having three windings, a line, an impulse transmitting device for transmitting impulses over said line, a first series circuit including the second of said windings and said trans-,

ter, a balancing network for side tone reduc- CJI tion including two impedances and another winding of the induction coil in series bridged acrosssaid transmitter, and a receiver circuit including a receiver and a third winding of the induction coil in series connected in a bridge of one of said impedances.

6. In a substation telephonecircuit, a trans mitter circuit including a transmitter and one winding of an induction coil, a balancing network for side tone reduction including two impedances and another winding of the induction coil in series connected in bridge of said transmitter, a receiver circuit including a receiver and a third winding of the induction coil connected in bridge of one of said impedances, means including said windings and connections responsive to the operation of said transmitter during speech transmission for causing two superimposed currents to flow in said receiver circuit.

7. In a substation. telephone circuit, a transmitter circuit including a transmitter and one winding of an induction coil, a balancing network for side tone reduction including two impedances and another winding of the induction coil in series connected in bridge of said transmitter. a receiver circuit including a receiver and a third winding of the induction coil connected in bridge ofone of said impedances, means including said windings and connections responsive to the operation of said transmitter during speech transmission for causing two superimposed currents to flow in said receiver circuit, said superimposed currents assisting or opposing each other dependent upon the connection of the receiver circuit to said one impedance.

8. In a substation telephone circuit comprising a transmitter circuit including a transmitter and one winding of an induction coil, a balancing network for side tonereduction including another winding of the induction coil, an impedance, a pure resistance, and a condenser in series connected in bridge of said transmitter. and a receiver circuit including a receiver and a third winding of r the induction coil 111 series connected in bridge of said impedance.

9. In a substation telephone circuit comprising a transmitter circuit including a transmitter and one winding of an induc- .tion coil in series, a balancing network for side tone reduction including another wind ing of the induction coil, a first nure resist-- v ance, a second pure resistance. and a condenser in series connected in bridge of said transmitter, and a receiver circuit including a receiver and a third winding of the induction coil in series connected in bridge of said first resistance.

10. In a substation telephone circuit com- I and a third winding of the'induction coil in series connected in bridgeof said first resist-g ance, the line impedance of the substation circuit for zero side tone beingsubstantiallv independent of frequency whenused on a line circuit having the same impedance.

11. In a substation telephone circuit comprising a transmitter circuit including a transmitter and one winding of an induction coil in's'eries, a balancing network for side tone reduction including another winding of the induction coil, an impedance, a tapped resistance, and a condenser in series connected in bridge of said transmitter, and a receiver circuit including a receiver and a third winding of the induction coil in series connected in bridge of saidimpedance.

EDMUND R. WIGAN. 

